The researchers found that the subjects from Mexico City had higher thresholds for detecting a smell, a poorer ability to describe smells, and a poorer ability to discriminate between smells than subjects from Tlaxcala, a state to the east of Mexico City that is geographically similar but has less pollution.

Mexico City is known for some of the world's most polluted air. The city of 25 million sits in a valley atop a high plateau, an ideal spot for smog to settle. Government figures suggest that the poor air quality may account for tens of thousands of premature deaths.

Officials have taken steps to address the problem. In 1989, the city introduced "Hoy No Circula," a program in which cars with license plates ending in certain numbers would be prohibited from driving on a given day of the week.